The application relates to a vibration sensor and a method for optimizing a piezoelectric drive for such a vibration sensor.
Various types of vibration sensors are known from the prior art. They are frequently used as vibration level switches with a diaphragm that can be caused to oscillate and a drive for oscillating the diaphragm and/or for tapping of an oscillation of the diaphragm and a mechanical oscillator arranged on the diaphragm, wherein frequently piezoelectric elements are used as drives. Such vibration level switches are used in particular for detecting fill levels or limit levels in containers for flowable and fluidizable media, in particular for fluids or bulk materials. Depending on the fill level in the container, the vibration level switches are in contact with the medium or not, such that a vibration frequency of the diaphragm or of the mechanical osciliator arranged on the diaphragm is influenced by the contact with the medium.
The vibration sensors known from the prior art are frequently operated with a bonded drive, wherein in the case of this type of drive, the piezoelectric element has at least two electrical contact structures in order for the disk-shaped piezoelectric element to interconnect a compensation element for adjustment of thermal expansion coefficients of the diaphragm and piezoelectric element to cause the diaphragm to oscillate and acquire osciliations of the diaphragms and convert them into a measurement signal. At least one first electrical contact structure is applied on an upper side of the piezoelectric element and at least one second electrical contact structure on a lower side of the piezoelectric element. Typically metallizations applied over the surface are used for contacting the piezoelectric element.
In the piezoelectric drives known from the prior art, as a rule contact is made over the entire surface of the lower side of the piezoelectric element, i.e. the surface of the piezoelectric element facing the diaphragm, while the surface of the piezoelectric element pointing away from the diaphragm of the piezoelectric element is provided either with one or with more electrical contact structures. By way of example, the upper side of the piezoelectric element can be quadruple segmented and in each case two segments arranged diagonally opposed can be in joint electrical contact. In this way it is possible using two of the contact structures to couple movement into the diaphragm and simultaneously to use the two other contact structures to acquire an oscillation resonance frequency and/or amplitude of the diaphragm. If the surface is not in segmented contact then stimulus and detection can only be carried out sequentially.
In the case of the piezoelectric elements that are typically used, a voltage is generated for detection which is proportional to a curvature of the piezoelectric element. Conversely, an applied voltage is converted to a curvature of the piezoelectric element proportional to an applied voltage.
In the vibration sensors known from the prior art the geometry and form of the electrical contact structure of the piezoelectric element are determined by experiments. The piezoelectric elements are typically designed in the shape of a disk and due to the manufacturing technology used, as a rule have a circular base area. As already mentioned, typically contact is made over the entire surface of the lower side of the piezoelectric element. Depending on the area of application it is further customary in the prior art to provide the upper side of the piezoelectric element either with an electrical contact structure over the entire surface or in multiple segments with a likewise approximately full surface contact structure. In sonic manifestations the lower side is not in contact.